1. Field of the Invention
The present invention is related to the field of bridge circuits and more particularly, to that portion of the field which is concerned with bridge circuits for eliminating the effect of a variable on an output signal. More particularly, the present invention is related to a bridge circuit which may be used in conjunction with a resistive sensor whose resistance varies as a function of two potentially interrelated parameters and which will render an output signal which varies only in response to a selected one of the parameters. The present invention is also related to the field of exhaust gas responsive engine control systems wherein a signal derived from an exhaust gas sensor is used to control the provision of air/fuel mixture to the engine.
2. Description of the Prior Art
The prior art teaches that an exhaust gas sensor may be positioned in the exhaust system of an internal combustion engine to monitor the combustion process and that the output signal from this sensor may be used to control the combustion process to maintain combustion at an optimum level whereat the deleterious exhaust emission products may be at an optimum value. For example, such a sensor may monitor the pressure of oxygen within the exhaust system as a measure of the quality of the combustion process and may control that combustion process by modulating, for example, the ratio of the air/fuel mixture to maintain that mixture at a selected ratio. This is of importance in the automotive industry which is being compelled to rely upon catalytic reactors or converters to treat the exhaust gases to arrive at mandated minimum values of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NO.sub.x). The most efficient and reliable catalytic converters require that the exhaust gas which enters the converter be the product of a combustion process which has occurred within a very narrow range of air/fuel ratios centered about a ratio which yields a particular exhaust gas chemistry. This ratio may be the stoichiometric ratio. The margin for error in the air/fuel ratio is very slight and exceeding this margin may result in catastrophic failure of the catalytic reactor or in grossly inefficient operation of the catalytic converter.
Extensive work is being undertaken in the area of the exhaust gas sensors and several sensors are known which, when inserted within the exhaust system, will generate a first relatively high level of signal when the combustion process occurs on one side of the stoichiometric ratio and a second relatively low level of signal when the combustion process occurs on the other side of the stoichiometric ratio. For example, the output signal may be high for mixtures rich in fuel content and may be low for mixtures which are lean in fuel content. The transition from the high level of signal to the low level of signal occurs as a virtual step at the stoichiometric ratio. Careful analysis of this output signal, particularly around the stoichiometric ratio, has demonstrated that the signal change is not in fact a step function but is a monotonically changing function from about the stoichiometric ratio minus two tenths of an air/fuel ratio to about the stoichiometric ratio plus two tenths of an air/fuel ratio.
For any given engine design and catalyst used therewith, the optimum engine performance, considering efficiency, economy and emissions, may be at an air/fuel mixture ratio which deviates from the stoichiometric ratio. For example, the engine design may inherently result in low output of one emissions component for example NO.sub.x so that the engine could be set to run slightly lean where more NO.sub.x may be generated, which is within the capability of the catalyst to convert, while improving economy. Thus determining the "desired ratio" requires taking into account the engine performance and the catalyst capabilities. At present, the preferable catalyst and most engines require an air/fuel mixture ratio which is slightly rich.
Control systems which respond to the output signal from such a sensor have been proposed to modulate the air/fuel ratio of the air/fuel mixture entering the engine. According to one proposal, a conventional carburetor would provide an air/fuel mixture having a grossly controlled air/fuel ratio less than stoichiometric and an auxiliary device would be provided, such as, for example a continuous flow fuel nozzle which would add additional quantities of fuel to render the total air/fuel mixture provided to the engine at the desired ratio. This continuous flow auxiliary nozzle may be for example a solenoid controlled needle valve positioned within a metering orifice. The quantity of fuel provided thereby would be controlled by the output signal of a system sensing the exhaust gases and variably energizing the solenoid to position the needle valve within the metering orifice to thereby add to or subtract from the quantity of fuel in the mixture being provided to the engine. According to another proposal, the system would control the position of an air valve whereby the quantity of air being admitted to the intake of the carburetor would be controlled to render the overall air/fuel mixture at the desired ratio. Still another proposal has involved the modulation of the scheduling of a scheduled electronic fuel injection system to continuously tailor the scheduling so that the quantity of fuel being provided to the engine would be exactly that necessary to provide a stoichiometric mixture for the combustion chambers of the engine.
Each of these systems relies upon establishing a mean value of sensor signal which would represent, for example, the stoichiometric ratio and then establishing error limits which would be within the range of acceptable combustion products for the associated catalytic reactor. For example, in a catalytic reactor that operates efficiently at 99 to 101% of the stoichiometric air/fuel ratio, the limits established by the control system could be at 99.5 and 100.5% of the stoichiometric air/fuel ratio so that when the output signal of the exhaust sensor reaches these limits, the device controlled by the system (e.g., the continuous flow orifice, the air valve or the scheduling for an electronic fuel injection system) would be actuated to increase or decrease the air/fuel ratio to cause the sensed exhaust gas signal to move away from the limit which it had approached toward the center value. This results in a system which has very tight tolerances and very small margin for error. This is, nevertheless, well within the capabilities of electronics to meet.
Copending commonly assigned patent application Ser. No. 198,515, now abandoned, "Air Fuel Ratio Sensing System" by H. L. Stadler et al. discloses an exhaust gas sensor of the resistive type which has been found to be most advantageous in examining exhaust gases for determining the air/fuel ratio of the combustion mixture. The sensor disclosed therein operates most efficiently, at least in an internal combustion engine environment, at temperatures within the range of from about 600.degree. C. to about 900.degree. C. Such a temperature range presents wide latitude in the placement of such a sensor within the exhaust system and also permits efficient sensor operation under all, or virtually all, conceivable engine operating conditions. Extensive testing with this type of sensor under all engine operating conditions has pointed out an area of significant difficulty with this type of sensor which has been determined to reside in the fact that the output signal of such a sensor includes a temperature dependent or parasitic component. This temperature dependent component can combine with (either add to or subtract from) the exhaust gas indicative signal component to skew the air/fuel ratio away from the stoichiometric ratio and, under extreme conditions, can result in air/fuel ratios being commanded which will exceed the limits which the associated catalytic converter or converters may efficiently accept. Maintaining the sensor at a precisely controlled temperature within its operating range would be one way of eliminating this temperature dependency problem. However, the exhaust system of an internal combustion engine is a highly dynamic environment and maintaining a constant temperature would be difficult and expensive and would require the provision of both heating and cooling mechanisms within the sensor, or alternatively placement at a relatively lower temperature environment and constant heating of the sensor. This would also entail complex sensing and control mechanism to ensure that the temperature of the sensor is maintained at a fixed and controlled level. This would greatly increase the cost of the sensor and would also represent a potential cause of failure since placement in a cooler region coupled with lack of heating as for example with a broken heater wire would cause the sensor to operate at a temperature environment at which it is most inefficient, for example below 600.degree. C. It is also an object of the present invention to provide an electrical system for temperature compensation of a sensor, which system is inexpensive and which is more reliable because it does not include failure-prone devices such as power transistors operated under marginal condition. It is also an object of the present invention to provide a means for eliminating temperature dependency which means may be efficiently incorporated within the overall control system.
According to the above noted copending commonly assigned patent application Ser. No. 198,515, now abandoned, the exhaust gas sensor may be advantageously constructed of a disc or pellet of ceramic titania. Such a material demonstrates a temperature dependent resistivity which is an exponential function of the absolute temperature. It is therefore a further object of the present invention to provide a circuit which may be combined with a variable resistance, which may be for example titania ceramic material, to provide an output signal which is temperature independent at least over a selected temperature range. In order to accomplish the last-mentioned objective, it is a further object of the present invention to provide a circuit which will firstly define a region of temperature dependency of the resistivity of the titania sensor in which the temperature dependency is a substantially linear function and which will combine the signal derived therefrom with a signal which demonstrates an opposite linearity so that the resultant signal becomes temperature independent.